Weather by Malcolm Riley

The Tacoma Narrows Bridge in Washington State USA opened in November 1940 and collapsed four months later.The effect of resonance on tides

Last November we looked at tides around the world and how, due to the natural sloshing backwards and forwards of the tide, some ocean areas get higher tides than others and some get little or no tide at all.
The example of the two ends of the Panama Canal having two different tide regimes was given.
Around our own coastline the rise and fall of tides vary considerably, at Fremantle close to the amphidromic point the tide range is small. At Broome the tide can range above nine metres. From the chart in that issue the open ocean tide is only a maximum of just over a metre.
Where does the other eight metres come from?
The answer is resonance. Resonance is where relatively small, well-timed forces can cause large fluctuations.
An example of this is soldiers breaking step when crossing a bridge. In 1831, 74 soldiers marching four abreast began to cross a bridge in Salford, England. The resonant frequency of their marching started a swaying motion that caused the bridge to collapse, with them on it. Luckily the river was shallow at the time and all survived.
A more dramatic example of this resonance was the Tacoma Narrows Bridge in Washington State USA. The bridge, opened in November 1940, collapsed four months later. The day the bridge collapsed the wind was blowing at around 60 kilometres per hour from the right direction that matched the resonance of the bridge and it began to ‘flutter’. So much so that after one hour the bridge collapsed.
Predicted Tide Range Burntcoat Head, Nova Scotia. Height in metres.The failure of this bridge was and still is a complex engineering and physics problem and it is mentioned only as a dramatic indicator of the application of small well-timed forces, not a definitive statement on collapsing structures. A good video can be found here
The incoming and outgoing tide have a regular period of 12 hours and 25 minutes. For coastal areas that have a resonance with this tide period due to their depth, length, width or shape the range of the tide will increase above the level that would occur in the adjacent open (deep) ocean.
In the example of the Panama Canal tides the Atlantic side had very little tide range due to the proximity of an amphidromic point. However, the Pacific side of the canal had a tide rang of five metres. This five metres consisted of just over a metre of open ocean tide with resonance effect due to the depth or shape of the bathymetry increasing the actual tide recorded to five metres.
The tides around northwestern Australia have resonance with the distance of the continental shelf from the land and the depth of the water.
The highest tide ranges in the world are in the Bay of Fundy, Nova Scotia, Canada. The Bay of Fundy has a natural resonance very close to the timing of the incoming and outgoing tide. One of the highest tides for 2010 is shown in the diagram at over 16m for Burncoat Head, Nova Scotia.
However, in nearby Halifax approximately 100kms away facing the Atlantic ocean, the tide range for the same period is about two metres.

*Malcolm Riley is the Public and Marine Officer for the Bureau of Meteorology in Hobart. He has worked in all States with the exception of Qld and is a Master V. He gives education courses on Marine Meteorology.